Emergency beacons operating at 406 MHz have seen worldwide acceptance over the past decade or so. The 406 MHz beacon population is over 250,000, and the Cospas-Sarsat (C-S) satellites and system infrastructure have seen significant improvements over the years. The C-S system is likely to be around for a long time. However, reference to C-S herein is intended to encompass any similar system which may replace the present C-S system within the term of this patent.
Although C-S requirements only addresses the 406 MHz portion of the beacons, most national authorities require the use of an auxiliary 121.5 MHz, low power, homing transmitter in all beacons. Prior to 406 MHz technology becoming widely available, 121.5 MHz-only beacons were used. The 121.5 MHz-only beacons are being phased out. Recently C-S decided to terminate the satellite processing of 121/243 MHz signals based on recommendations from the International Maritime Organization (IMO) and the International Civil Aviation Organization (ICAO). This means that existing 121.5 MHz-only beacon users have to switch to a 406 MHz beacon.
Currently, to the best of applicant's knowledge all 406 MHz beacons have a 121.5 MHz transmitter that is used as a homing signal for aiding search and rescue (SAR) aircraft for homing the last few kilometres into the beacon. Technical problems, such as set out below, experienced during development of these beacons have mostly been overcome, but the developments have added cost and complexity to the beacon. Operational problems, such as when a user decides to hold his emergency position indicating radio beacon (EPIRB) instead of allowing it to float, still cause problems.
When an activated EPIRB is held by a survivor in a life raft, the transmitted signal may be attenuated significantly enough to preclude reception by the SAR airplane or helicopter. This defeats the purpose of having a homing signal in the beacon. The problem arises from the fact that most EPIRBs have an antenna that is optimized for 406 MHz operation, thus resulting in an electrically short antenna at 121.5 MHz which is very narrowband and very dependent on the water, which is acting as the antenna's ground plane, for proper radiation characteristics. When it is operated out of the water, the antenna is detuned (presenting a severe mismatch to the output power amplifier) and the ground plane effect is removed resulting in a reduction in the radiated signal. As beacons have become physically smaller over the years, this problem has been made worse. The 121.5 MHz homing transmitter in 406 beacons has been the cause of many problems. Such problems range from designing to meet the multitude of stringent signal parameters to the generation of harmonics causing interference problems in location protocol beacons. In the case of emergency locator transmitters (ELTs), energy from the aircraft VHF radio would be rectified in the unpowered ELT's 121.5 MHz output stage and produce an interfering signal for the aircraft's global positioning system (GPS) receiver. Overcoming these problems has added cost and complexity to 406 beacons. Problems have plagued 121.5 MHz beacons even long before the introduction of the superior 406 MHz technology.